Large flake reconstituted mica insulation

ABSTRACT

A reconstituted mica insulation sheet comprising large mica flakes has improved physical properties, particularly resistance to cut-through and sever mechanical abuse without danger of electrical failure. The sheet can be impregnated with resin, laminated to webs, adhesive-coated, slit, and coiled to form insulating tapes.

C United States Patent l1ll3,618,753

[72] Inventor David W. Gllsspoole 5 Rein-g g cu Stlllwater, Mlnn. [2 PPNo. 760.356 UNITED STATES PATENTS [22] Filed Sept. 1968 2.739.638 3/1956Lewls et a1. 161/163 X [45] Patented "NJ, 3,001,571 9/1961 Hatch.161/163 Assigncc ad Mnu'actur'n' HeaslCy.

Camp, 3.117.900 1/1964 Jones W 161/163 sim Flu. Mum. 3.131.114 4/1964Heyman 161/193 3.215.590 11/1965 Purvis. 7. 161/163 3.226.286 12/1965Scheuer 161/163 Primary Examiner-John T. Goolkasian [54] LARGE FLAKERECONSTITUTED M'CA ASSISIIHII Examiner-George W MOXOII. ll

INSULATION Auorne K|nney. Alexander. Sell. Steldt & Delahunt 9 Claims,No Drawings [52] U.S. C1. 1 t 1 .1 v 206/59. ABSTRACT: A reconstitutedmica insulation sheet compris- 161/163. 161/171 ing large mica flakeshas improved physical properties. parlSl] lnt.Cl........ B32b'l9/00.ticularly resistance to cut-through and sever mechanical abuseB32b19/02,B65d85/67 without danger of electrical failure. The sheet canbe im- [50] Fleld of Search 161/163. pregnated with resin. laminated towebs. adhesive-coated, slit,

171; 206/59 and coiled to form insulating tapes.

BACKGROUND OF THE INVENTION This invention relates to reconstituted micainsulation. More particularly, it relates to reconstituted micainsulation sheets and tapes having improved physical properties,particularly resistance to cut-through and severe mechanical abuse.

Cut-through of electrical insulation occurs when, during application oruse, a sharp edge or corner of an insulated part forces its way throughand physically separates the insulation, causing electrical failure. Aparticularly acute cut-through problem exists in which voltage turbinegenerators having rectangular copper conductors which must be insulatedfrom each other and from ground. The insulation utilized must haveexceptional electrical and mechanical properties, particularlyresistance to cut-through and severe mechanical abuse, and mustwithstand being wrapped around small angle bends and being forced intotight crevices without cracking, splitting, or developing voids whichwould cause electrical failure. It has been found that resistance tothis type of failure can be predicted from the results of a laboratorytest which measures the force, in pounds, required to force a sharp edgethrough an electrical insulation.

Mica has excellent electrical, mechanical, and thermal properties, butis thick and inflexible in its naturally occurring state. Laminar micacrystals have been manually delaminated into noncohesive splittings,laid in overlapping pattern by hand or machine, and bonded with resinousmaterial to form an insulation sheet having resistance to mechanicalabuse and cut-through adequate for most purposes. However, machine laidsheets are thick, often discontinuous, have a tendency to flake,are notuniform and must be used in thick layers to obtain adequate electricalproperties. Hand-laid insulation sheets, are expensive, difficult tomake, have a tendency to flake, are in short supply domestically andconsequently must be obtained from import sources.

U.S. Pat. Nos. 2,405,576, 2,549,880, and 2,614,055 disclosereconstituted mica insulation sheets made by reducing naturallyoccurring mica into thin cohesive flakes, in the absence of adeactivating atmosphere, pressing the flakes together, and drying underheat and pressure. U.S. Pat. No. 3,l3l,l l4 discloses the broad flakesize range, about 3.5 to 400 mesh, utilized in reconstituted mica flakeinsulation sheets having glass flakes included therein. Such prior artreconstituted mica insulation sheets have good electrical properties andare considerably more flexible and uniform than sheets of micasplittings, but are opaque and are dependent upon an impregnating resinfor resistance to cut-through and to physical abuse. When impregnatedwith soft resin the sheets lack resistance to physical abuse anddeformation while sheets impregnated with hard resin crack and splitduring use with a resultant decrease in electrical insulationproperties.

Despite the long-recognized desirability of mica sheet or tapeinsulation having both the resistance to cut-through and physical abuseof mica splittings, and the electrical superiority, uniformity, lowcaliper and flexibility of reconstituted mica, such a product has neverheretofore existed.

SUMMARY This invention provides reconstituted mica insulation sheets andtapes that combine excellent resistance to cut-through and physicalabuse with the electrical superiority, uniformity, low caliper, andflexibility of reconstituted mica.

Insulation sheets and tapes prepared in accordance with this inventionhave excellent electrical and physical properties. They are flexible,dense, uniform, continuous, translucent, nonflaky, and retain theirelectrical properties such as are and corona resistance, dielectricstrength and low power factor, in severe use. Further, these sheets areideally suited for insulating sheets, wrappers, and tapes'havingsubstantially improved resistance to cut-through and mechanical abuseduring application and use. The insulation sheets of the invention aread mirably suited for commercial use in insulating direct currenttraction motors, alternating current motors, and transformers, and areparticularly well suited for insulating the rectangular conductors ofhigh-voltage turbine generators. These mica insulation sheets can bebent around sharp angle bends, including the right angle bends at theedges of rectangular stator conductors, and can be forced into smallcrevices without danger of electrical failure.

Surprisingly, it has been discovered that a reconstituted micainsulation sheet can be made from large mica flakes so as to have aresistance to cut-through, when unimpregnated, more than 200 percentgreater than broad flake size prior art reconstituted mica sheets, whileretaining the advantageous uniformity, flexibility, and electricalproperties. The mica flakes used to make these sheets are considerablylarger than the average size of mica flake used to make prior artsheets. At least 40 percent by weight of the flakes are larger than l4mesh, at least 70 percent are larger than 35 mesh and at least percentlarger than 60 mesh. Preferably, at least 50 percent of the flakes arelarger than l4 mesh, at least 80 percent larger than 35 mesh and atleast percent larger than 60 mesh. Prior art reconstituted micainsulation sheets utilize a broad flake size distribution from about 3.5to 400 mesh, primarily from about 35 to 400 mesh. It is believed thatthe large surface area of individual flakes permits a large overlap areawhich increases cohesion between flakes, eliminates internal voids inthe sheet to increase the specific gravity and provides a continuoussheet. it is also thought that the large surface area of the largeflakes contributes the excellent resistance to cutthrough and physicalabuse.

The strength and resistance to abuse of mica insulation sheets areillustrated by tensile strength (ASTM Test D-828 Prior art insulationsheet made with a broad flake size range (Acim Brand) has an averagetensile strength of about 400-500 p.s.i. Prior art insulation sheet madewith very small flakes (Samica Brand) has an average tensile strength ofabout l,200-2,400 p.s.i., while sheets made according to this inventionhave an average tensile strength of about 3,5005,000 p.s.i.

The freedom from internal voids which characterizes the sheet of thisinvention is illustrated by its specific gravity of about l.6-2.5, whichapproaches the 2.7-2.8 specific gravity of solid muscovite. Prior artreconstituted mica insulation sheet has a specific gravity of about0.9-1.5, which clearly indicates the large number of voids containedtherein. The specific gravity of mica insulation sheets is readilydetermined by the Mercury Intrusion Method described in Bulletin 2405-Aof the American Instrument Company.

Muscovite and phlogopite mica larger than about 3 mesh can be utilizedto produce these large mica flakes. After a preliminary water washing toremove dirt and debris, the mica blocks are split by means of water jetsstriking the mica blocks at an angle substantially parallel to the planeof cleavage as disclosed in U.S. Pat. No. 2,405,576. The flakes areclassified with the proper mesh in U.S. standard sieve and reconstitutedby standard papermaking techniques into a mica insulation sheet havingan overlapping arrangement of large mica with their surface incontiguous relation. Flakes split in this manner are very thin and havea large surface area as compared to their thickness. When reconstituted,the individual mica flakes adhere to each other by natural cohesiveforces, as contrasted with mica splittings which must be bonded togetherwith resin.

After the reconstituted mica insulation sheet is made, it may beimpregnated with inorganic resin such as boron phosphates and potassiumborates and organic resin such as shellac, epoxy, alkyd, polyester,silicone, etc., the choice of resin depending on a balance of cost,temperature resistance, flexibility, and electrical resistance required.Inorganic or organic binders, fibers, and filaments may be incorporatedinto the sheet, if desired. For some industrial applications, it isdesirable to laminate the large flake impregnated mica insulation sheetto a web such as polymeric film and woven or nonwoven fabrics. either byusing the impregnating resin as an adhesive or by applying a separateadhesive between the web and the insulation sheet. The improvedcut-through resistance of large flake reconstituted mica insulationsheet is not dependent upon the saturating resin or backing web, wherebypermitting the utilization of numerous soft resins which retain theflexibility of the insulation sheet.

Resistance to cut-through is determined by installing a W x V2" x 3"mild steel bar, having a 32 micro inch finish on all sides, in each jawof an lnstron" tensile tester containing a compression cell. Both barsare installed on edge at right angles to each other so that only pointcontact will occur when the bars touch. The sample specimen is placedbetween the bars and the machine jaws closed at the rate of 0.5inch/minute. The right-angle edges of the steel bars are forced throughthe mica sheet until they contact each other closing an electricalcircuit to light a bulb which indicates when cutthrough is complete. Thelnstron" recorder provides an accurate reading of the force, in pounds,required to force the sharp edges through the mica sheet.

The following examples, in which all parts are by weight unlessotherwise noted, illustrate preparation of the insulation sheets andtapes of this invention without limiting the scope thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 This exampledescribes splitting mica-blocks into flakes and making a reconstitutedlarge flake mica insulation sheet. lndian muscovite punch scrap is splitinto thin flakes by subjecting it to jets of water as described in U.S.Pat. No. 2,405,576. The slurry of wet mica flakes was classified bywashing the flakes through a series of U.S. standard screen sieves, allflakes below 140 mesh being discarded. The classified flakes arerecombined in a 1 percent mica slurry in water such that 52 percent byweight of the flakes in the final slurry are larger than 14 mesh, 85percent are larger than 35 mesh and 93 percent are larger than 60 mesh.

The reconstituted sheet was then made by using commercial paper millequipment comprising, in connected series, an agitator, a storage chest,and a cylinder-type paper machine having an endless wet press belt whichtransfers the wet mica flake layer from the cylinder screen to a steelwet press roll. The speed of the cylinder screen was 16 ft./minute. Anendless woven cotton belt transferred the wet paper web into a dryingsection of the machine where the paper was dried by passing through aseries of steel cylinders heated to between about 140 F. and 180 F. Thedelivery rate of mica flake slurry to the paper machine determines thethickness of the insulation sheet obtained. Sheet thickness can bevaried from about 0.5 mil to about 30 mils. lf thicker sheets aredesired, sheets can be laminated or sandwiched together. Mica insulationsheets produced in this manner have an unusually high degree ofuniformity of flake structure as is evident from the uniformtranslucency observed when a sheet is held up to a light source.

An insulation sheet made in the aforedescribed manner had an averagethickness of 2.45 mils when tested according to ASTM Test D-374, MethodC, a tensile strength of about 4,000 p.s.i. when tested according toASTM Test D-838, a dielectric strength of about 643 volts per mil whentested according to ASTM Test D-l49, a cut-through resistance of about 7lbs., a specific gravity of about 1.7, and provided a useful insulationsheet without impregnation. This mica sheet was impregnated, laminated,coiled and slit as described in examples 2-5.

An insulation sheet utilizing the broad 3.5 to 400 mesh flake sizedistribution of the prior art, disclosed by U.S. Pat. No. 3,13 1,1 14,was made and tested in the same manner. It has an average thickness of2.85 mils, a tensile strength of about 200 p.s.i., dielectric strengthof about 350 volts per mil, a cutthrough resistance of about 2 lbs., anda specific gravity of about 1.2.

EXAMPLE 2 This example describes impregnation of a portion of the largeflake reconstituted mica insulation sheet of example 1 with apolyester/epoxy resin. A mixture of parts of polyester/epoxy resin and 1part of tertiary amine [His-(2,4,6- dimethyl amino methyl)-phenol] wasprepared according to example 2 of U.S. Pat. No. 3,027,279. Theimpregnating resin was diluted to 25 percent solids with methyl ethylketone and applied to the mica sheet by means of a conventional dip andflow method. The impregnated paper was then dried at F. for about 15minutes and subsequently cured for 10 minutes at 400 F. The resultantinsulation sheet was tough, flexible, and in a fully cured state. Theresin content of the impregnated sheet was determined by weighing asample both before and after coating and was found to be about 20percent by weight.

The impregnated sheet, when tested in the manner of example l, was foundto have an average thickness of 4.4 mils, a tensile strength of about10,400 p.s.i., a dielectric strength of about 690 volts per mil, and acut-through resistance of about 7.7 lbs.

lmpregnation of a reconstituted mica insulation sheet utilizing thebroad 3.5 to 400 mesh flake size distribution of the prior art in asimilar manner with the same resin produced a sheet that was 4.7 milsthick, contained 20 percent resin and had a cut-through resistance of3.6 lbs.

EXAMPLE 3 A portion of the reconstituted large flake mica insulationsheet of example 1, was impregnated in the manner of example 2, with anisooctyl acrylate/acrylic acid/epoxy terpolymer resin and 0.45 percenturanyl nitrate hexahydrate catalyst.

The copolymer resin was made by first mixing 1,997.5 pounds of isooctylacrylate, 29.0 pounds of acrylic acid, and 17.9 pounds of tertiarydodecyl mercaptan in a stainless steel tank. A charge of 2,787.5 poundsof toluene was then placed in a 1,500 gallon glass-lined kettle, afterwhich 147 pounds of the mixture in the tank was added. The kettle waspurged with nitrogen and heated to F. with constant agitation, all thewhile maintaining a slight nitrogen flow through the kettle. Next, threeseparate 22-pound charges of azo-bisisobutyronitrile dissolved intoluene were added at equal intervals over a period of approximately 50minutes; each charge being 22 percent solids. During this period themixture from the tank was continuously added at a rate of about 37pounds per minute. After waiting another 50 minute period, 20 pounds of20 percent azo-bis-isobutyronitrile in toluene was added and thetemperature maintained at 175 F. for 2V2 additional hours. Then, 124.1pounds of 3,4-epoxycyclohexylmethyl-3,4- epoxy-cyclohexanecarboxylatehaving an average molecular weight of about 260 and a viscosity of about500 centipoises at 24 C. (Union Carbide ERL-4221 was added andthoroughly mixed. The mica sheet was impregnated as in example 2, andafter drying for 10 minutes at 220 F., and curing 10 minutes at 400 F.,the resin content was determined to be 20 percent. The tacky impregnatedinsulation sheet was then joined to a 1.5 mil nonwoven web of heatbonded polyethylene terephthalate fibers by means of laminating rolls.

The cut-through resistance of this laminate is substantially greaterthan that of a prior art mica insulation sheet, as described in example1, similarly impregnated and laminated.

EXAMPLE 4 This example illustrates the impregnation of a portion of themica insulation sheet of example 1 with soft flexible silicone resin bylamination to a resin impregnated woven glass cloth and subsequentlamination to polyethylene terephthalate film.

Polysiloxane resin (General Electric SR-32) was diluted to 35 percentsolids in toluene. A 2 mil woven glass cloth was impregnated with thisresin by the common dip and flow method and laminated to a portion ofthe mica sheet of example 1 by means of laminating rolls, and thelaminate dried for 4 minutes at 400 F. The laminate contained percentresin, was 7.5 mils thick, and had a cut-through resistance superior tothat of a prior art mica insulating sheet, as described in example l,similarly saturated and laminated.

The unlaminated exposed mica surface was then coated with the siliconeresin saturant disclosed above, by reverse roll coating technique, anddried for 4 minutes at 250 F. The tacky mica surface was then laminatedto 0.25 mil biaxially oriented polyethylene terephthalate film by meansof laminating rolls, coiled into a jumbo roll, and subsequently slitinto Y4 wide rolls of tape. The resin content of the laminate was l5percent and the cut-through resistance was superior to that of a priorart mica insulating sheet, as described in example I, similarlyimpregnated and laminated.

EXAMPLE 5 This example illustrates the lamination of a polyester filmweb to the impregnated large flake reconstituted mica sheet of example2. After impregnation and following drying at l50 F. for minutes, theimpregnated mica sheet was cooled and laminated to 0.5 mil biaxiallyoriented polyethylene terephthalate film by means of rotating pressurerolls. The cut through resistance of the laminate was superior to thatof a prior art mica insulating sheet, as described in example I,similarly impregnated and laminated.

lclaim:

1. in a self-supporting reconstituted mica insulation sheet comprisingan overlapping arrangement of unimpregnated mica flakes, the improvementcomprising:

the mica flakes having at least 40 percent by weight of the individualflakes larger than 14 mesh,

at least 70 percent by weight of the individual flakes larger than 35mesh, and

at least 90 percent by weight of the individual flakes larger than mesh,whereby the insulation sheet has increased density, excellent tensilestrength, outstanding electrical properties and resistance tocut-through and physical abuse.

2. The insulation sheet of claim 1 impregnated with resin.

3. The insulation sheet of claim 2 wherein the resin selected from theclass consisting of epoxy resin, polyester resin, silicone resin, alkydresin, and acrylic resin.

4. The insulating sheet of claim 3 laminated to a fibrous web.

5. The laminate of claim 4 wherein the web is polyethyleneterephthalate.

6. The insulation sheet of claim 3 laminated to a polymeric film.

7. The laminate of claim 6 wherein the polymeric film is biaxiallyoriented polyethylene terephthalate.

8. As a new article of commerce, an insulating tape wound convolutelyupon itself in roll form and capable of being unwound therefrom withoutdelaminating or ofl'setting, said tape comprising the insulation sheetof claim 1 or 2 laminated to one side of a thin flexible web, wherebysaid insulating tape has increased specific gravity, excellent tensilestrength. outstanding electrical properties, and resistance tocut-through and physical abuse.

9. The tape of claim 8 wherein the insulation sheet has a second thinflexible web laminated on its other side.

2. The insulation sheet of claim 1 impregnated with resin.
 3. Theinsulation sheet of claim 2 wherein the resin selected from the classconsisting of epoxy resin, polyester resin, silicone resin, alkyd resin,and acrylic resin.
 4. The insulating sheet of claim 3 laminated to afibrous web.
 5. The laminate of claim 4 wherein the web is polyethyleneterephthalate.
 6. The insulation sheet of claim 3 laminated to apolymeric film.
 7. The laminate of claim 6 wherein the polymeric film isbiaxially oriented polyethylene terephthalate.
 8. As a new article ofcommerce, an insulating tape wound convolutely upon itself in roll formand capable of being unwound therefrom without delaminating oroffsetting, said tape comprising the insulation sheet of claim 1 or 2laminated to one side of a thin flexible web, whereby said insulatingtape has increased specific gravity, excellent tensile strength,outstanding electrical properties, and resistance to cut-through andphysical abuse.
 9. The tape of claim 8 wherein the insulation sheet hasa second thin flexible web laminated on its other side.